The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 2001-304454 filed Sep. 28, 2001; and
Japanese Patent Application No. 2001-304524 filed Sep. 28, 2001.
1. Field of the Invention
The present invention relates to a compact zoom lens system having a zoom ratio about two to four, a small amount of chromatic aberration, and high optical performance across the entire image, suitable for a video camera and an electronic still camera using an imaging device such as a solid state imaging device.
2. Related Background Art
A lot of zoom lens systems suitable for a video camera and an electronic still camera using an imaging device such as a solid state imaging device have been proposed. Among them, the majority of the zoom lens systems having a zoom ratio about two to four is a zoom lens type having a negative lens to the most object side of the lens system. For example, there is a zoom lens system disclosed in Japanese Patent Application Laid-Open No. 11-23967.
In recent years, the number of image pixels used in a video camera and an electronic still camera using an imaging device such as a solid state imaging device has steadily been increasing. In accordance with the trend, the lens system has been requested to have higher optical performance across the entire image. In particular, the lens system has been requested to reduce flare component and chromatic aberration as well as have high optical performance in the periphery.
However, the zoom lens systems disclosed in Japanese Patent Application Laid-Open No. 11-23967 and the like have had a problem not to have sufficient ability to reduce flare component or chromatic aberration across the entire image.
The present invention is made in view of the aforementioned problem and has an object to provide a zoom lens system having a zoom ratio about two to four, a small amount of chromatic aberration, and high optical performance across the entire image, suitable for a video camera and an electronic still camera using an imaging device such as a solid state imaging device.
According to one aspect of the present invention, a zoom lens system includes, in order from an object along an optical axis, a first lens group G1 having negative refractive power, and a second lens group G2 having positive refractive power. A distance between the first lens group G1 and the second lens group G2 decreases when the state of lens group positions varies from a wide-angle end state W to a telephoto end state T. The second lens group G2 is composed of, in order from the object, a first lens U21 having positive refractive power, a first cemented lens U22 having negative refractive power, and a second cemented lens U23 having positive refractive power. The first cemented lens U22 in the second lens group G2 is constructed by a positive lens U22P cemented with a negative lens U22N. The second cemented lens U23 in the second lens group G2 is constructed by a negative lens U23N cemented with a positive lens U23P.
In one preferred embodiment of the present invention, the zoom lens system further includes a third lens group G3 having positive refractive power located to an image side of the second lens group G2. The third lens group G3 is substantially fixed relative to the image plane when the state of lens group positions varies from the wide-angle end state to the telephoto end state.
In one preferred embodiment of the present invention, the following conditional expressions are satisfied;
0.80 less than f2/|f22| less than 1.70xe2x80x83xe2x80x83(1) 
1.40 less than f23/|f22| less than 3.50xe2x80x83xe2x80x83(2) 
where f2 denotes the focal length of the second lens group G2, f22 denotes the focal length of the first cemented lens U22 in the second lens group G2, and f23 denotes the focal length of the second cemented lens U23 in the second lens group G2.
In one preferred embodiment of the present invention, the following conditional expressions are satisfied;
n23Nxe2x88x92n23P greater than 0.13xe2x80x83xe2x80x83(3) 
xcexd23Pxe2x88x92xcexd23N greater than 15xe2x80x83xe2x80x83(4) 
where n23P denotes the refractive index at d-line (xcex=587.6 nm) of the positive lens U23P in the second cemented lens U23 in the second lens group G2, n23N denotes the refractive index at d-line of the negative lens U23N in the second cemented lens U23 in the second lens group G2, xcexd23P denotes Abbe number of the positive lens U23P in the second cemented lens U23 in the second lens group G2, and xcexd23N denotes Abbe number of the negative lens U23N in the second cemented lens U23 in the second lens group G2.
In one preferred embodiment of the present invention, at least one surface of the first lens U21 in the second lens group G2 is an aspherical surface having the shape that the refractive power becomes weak as the height separates from the optical axis.
According to another aspect of the present invention, a zoom lens system includes, in order from an object along an optical axis, a first lens group G1 having negative refractive power, and a second lens group G2 having positive refractive power. A fixed stop FS (by the term xe2x80x9cfixed stopxe2x80x9d in this specification is meant a stop whose aperture diameter is not variable) is arranged between the first lens group G1 and the second lens group G2. A distance between the first lens group G1 and the second lens group G2 decreases when the state of lens group positions varies from a wide-angle end state W to a telephoto end state T. When the state of lens group positions varies from the wide-angle end state W to a certain intermediate focal length state M, a distance between the first lens group G1 and the fixed stop FS decreases as the fixed stop FS is moved with the second lens group G2 in a body. When the state of lens group positions varies from the certain intermediate focal length state M to the telephoto end state T, a distance between the second lens group G2 and the fixed stop FS decreases as the fixed stop FS is moved with the first lens group G1 in a body.
In one preferred embodiment of the present invention, the second lens group G2 has an aperture stop S and the following conditional expressions are satisfied;
0.80 less than fM/(fWxc2x7fT)xc2xd less than 1.30xe2x80x83xe2x80x83(5) 
0.25 less than D2W/(D1W+D2W) less than 0.65xe2x80x83xe2x80x83(6) 
xcfx86FSxc2x7FNOT/fT greater than 1.40xe2x80x83xe2x80x83(7) 
xcfx86FS/xcfx86ST less than 1.20xe2x80x83xe2x80x83(8) 
where fW denotes the focal length of the zoom lens system in the wide-angle end state W, fM denotes the focal length of the certain intermediate focal length state M, fT denotes the focal length of the zoom lens system in the telephoto end state T, D1W denotes the distance between the most image side surface of the first lens group G1 and the fixed stop FS in the wide-angle end state W, D2W denotes the distance between the fixed stop FS and the most object side surface of the second lens group G2 in the wide-angle end state W, FNOT denotes f-number of the zoom lens system in the telephoto end state T, xcfx86FS denotes the diameter of the fixed stop FS, and xcfx86ST denotes the maximum diameter of the aperture stop S in the telephoto end state T.